This invention relates generally to semiconductor devices, and more particularly, to a transistor structure which alleviates problems associated with high base resistance in the narrow quantum well, or band gap, base region of certain transistor structures, by creating bound states in the quantum-well by using a base material having a lower band gap than the contact layers.
Resonant tunneling devices were first investigated by L. Chang et al. 24 Applied Physics Letters 593 (1974), the quantum well characteristics of such devices being applicable to two-terminal and three-terminal semiconductor devices. Generally, resonant tunneling is achieved when a bias potential across a plurality of terminals is established such that one of the bound states in the quantum well has the same energy level as the Fermi level of the input electrode. It is a significant advantage of semiconductor devices which operate under the resonant tunneling regime that very fast charge transfer is achieved, resulting in generally excellent high speed switching characteristics for the devices.
It is a problem with three-terminal tunneling semiconductor devices that a tunneling leakage current between the base and collector regions substantially diminishes the current gain of the device. The obvious approach to this difficulty is to increase the width of the barrier between the base and collector. However, such increased barrier width also tends to decrease the magnitude of the desired emitter-to-collector tunneling current. There is therefore a need for a three-terminal semiconductor device wherein high current gain is maintained without significantly impeding emitter-to-collector tunneling current.
It is, therefore, an object of this invention to provide a transistor structure which reduces undesirable base-to-collector tunneling.
It is another object of this invention to provide a transistor structure which improves transistor gain without reducing desired emitter-to-collector resonant tunneling current.
It is also an object of this invention to provide a resonant-tunneling transistor structure which exhibits high current gain.
It is a further object of this invention to provide a transistor structure which alleviates the problem of high base-to-collector tunneling current as bias is applied.
It is additionally an object of this invention to provide a semiconductor device which retains a high switching speed while affording reduced leakage current.
It is yet a further object of this invention to provide a resonant-tunneling transistor which affords a reduced base-to-collector capacitance.
It is also another object of this invention to provide a transistor structure which affords an equipotential base region for the application of bias.
It is yet an additional object of this invention to provide a transistor structure in which base current does not tunnel to the collector in significant measure.
It is still another object of this invention to provide a transistor structure which exhibits reduced base resistance.